A malaria vaccine that blocks infection by Plasmodium falciparum with high efficacy would also prevent malaria transmission, improving prospects for regional malaria elimination and eventual global eradication. Immunization of humans by exposure to mosquitoes carrying radiation-attenuated P. falciparum sporozoites protects >90% of volunteers against experimental P. falciparum challenge. Our collaborators at Sanaria Inc. have developed a manufacturing process for obtaining purified P. falciparum sporozoites for human administration by needle injection. This product, the PfSPZ Vaccine, is produced in aseptically raised adult Anopheles stephensi mosquitoes fed on blood cultures containing aseptic P. falciparum gametocytes. After infected mosquitoes are irradiated to attenuate the sporozoites, these radiation-attenuated sporozoites are isolated, purified, and the final product is cryopreserved. A recent study found that five intravenous (IV) doses of the PfSPZ Vaccine had 100% protective efficacy in the highest dose group. Obstacles that must be overcome to establish the utility of this vaccine include assessment of its safety and efficacy against genetically diverse natural parasites in malaria-endemic populations, and diminishing the number of doses required to achieve high protection. We aim to advance the clinical development of this vaccine in a randomized, double-blind, placebo-controlled dose escalation Phase 1 clinical trial in Balonghin, Burkina Faso, where malaria is endemic. To evaluate the importance of total sporozoite dose and number and timing of doses to immunogenicity and efficacy in a population with pre-existing antimalarial immunity, we will use twice the total cumulative dose that provided 100% efficacy in the NIH study to compare the same dosing schedule used in that study against three doses spaced either 8 or 12 weeks apart. Ninety healthy adults will be enrolled into three cohorts of 30, and randomized in a 2:1 ratio within each cohort to receive escalating doses of PfSPZ or placebo. Safety and tolerability, humoral and cell-mediated immunity, and exploratory measures of overall and strain-specific efficacy will be evaluated. This will be the first clinical trial to establish safety and tolerability of this promiing malaria candidate vaccine in malaria-experienced individuals living in a malaria-endemic area, and the first evaluation of the efficacy of PfSPZ Vaccine against diverse P. falciparum strains that occur in nature. If successful, this trial will advance the clinical development of a vaccine intended to eliminate malaria from geographically defined areas through mass immunization and to prevent malaria in non-immune visitors to malarious areas, such as tourists and business, aid, diplomatic and military personnel. The exploration of immunological and parasite genomic endpoints will advance our understanding of vaccine-induced protective immunity to malaria and help to determine whether and how a multi-strain sporozoite vaccine should be developed to provide broad protection against genetically diverse malaria parasites.